not sure where the standard location is in the rv-12 .. but in the rv-12iS, it is under the baggage floorboard.. accessible thru an access panel in the floor.
sounds like your custom fuel tank would be in the way of the floor... my old rv-6 had it mounted on top of the battery box, which was located in the cabin, against the firewall and floorboard.. between the pilot and copilot's rudder pedals. when i switched it over to a 406Mhz unit, i moved it to the empennage, on the floor.